The difficult search for organocerium(iv) compounds†
This Tutorial Review provides an overview of the historic and current development of the organometallic chemistry of cerium in its oxidation state 4+. Among the tetravalent lanthanide ions, only Ce4+ forms stable coordination compounds (e.g. (NH4)2[Ce(NO3)6]). Important fields of applications for cerium(IV) compounds include organic synthesis, bioinorganic chemistry, materials science, and industrial catalysis. In sharp contrast, organometallic cerium(IV) compounds are still exceedingly rare. The history of organocerium(IV) compounds is an exciting story of ups and downs. The so-called cerocene (= bis(η8-cyclooctatetraenyl) cerium) has been known since 1976. Other early reports e.g. about Cp4Ce (Cp = η5-cyclopentadienyl), were later disproven. However, significant progress in this field has been made in recent years through the use of carefully designed ligands and more sophisticated synthesis protocols. Taking the case of organocerium(IV) chemistry, this Tutorial Review also tries to exemplarily show how difficult synthetic and theoretical problems can eventually be solved through newly designed synthesis strategies (e.g. as accomplished for cyclopentadienyl and carbene derivatives) and a rewarding collaboration between synthetic and theoretical chemists (cf. the cerocene problem).